This might be a dumb question (and answered many times), but does a Dark Urge character have to look like that stupid lizard thing? Makes me think of the Gorn from Star Trek.
Personally I think some people are taking the idea of "canon" for this character way too seriously given that so far Larian have not declared anything canon for this game(and likely never will) but even if they did it would most likely not be the dark urge. So play the origin however you like with whatever race and class. On the official website Larian also list the Dark Urge as a "Sorcerer" and that seems like a really poor choice that doesn't fit the origin story and the theme of the rewards from its questline as we know so far. So I really wouldn't take the white dragonborn and sorcerer as anything more than Larian trying to give the character a striking look for promotional material so people would know which of their promotional stuff was intended to show the Dark Urge.
